Dispersion spun fluoropolymer fiber prepared from non-melt-processible polytetrafluoroethylene and perfluoroalkoxy

ABSTRACT

A dispersion spun fluoropolymer fiber prepared from non-melt-processible polytetrafluoroethylene particles and perfluoroalkoxy particles.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Patent Application No. 61/635,521, filed on Apr. 19, 2012, and tilted, “Dispersion Spun Fluoropolymer Fiber Prepared from Non-Melt-Processible Polytetrafluoroethylene and Perfluoroalkoxy,” and U.S. Provisional Patent Application No. 61/635,721, filed on Apr. 19, 2012, and titled “Flexible Laminate Structure,” the entire contents of which are incorporated herein by reference.

FIELD OF INVENTION

The present invention is directed to a dispersion spun fluoropolymer fiber, and more particularly, to a dispersion spun fluoropolymer fiber prepared from non-melt processible, high molecular weight, polytetrafluoroethylene particles and perfluoroalkoxy particles.

BACKGROUND OF INVENTION

Dispersion spun or wet polytetrafluoroethylene (PTFE) yarns are typically produced by forming a spin mix containing an aqueous dispersion of poly(tetrafluoroethylene) particles and a solution of a cellulosic ether matrix polymer. The spin mix is then extruded at relatively low pressure (e.g., less than 150 pounds per square inch) through an orifice into a coagulation solution usually containing sulfuric acid to coagulate the matrix polymer and form an intermediate fiber structure. The intermediate fiber structure, once washed free of acid and salts, is passed over a series of heated rolls to dry the fiber structure and sinter the PTFE particles into a continuous PTFE filament yarn. Sintering the intermediate PTFE fiber structure causes the PTFE particles in the structure to coalesce and entangle thus forming a continuous PTFE filament fiber.

SUMMARY OF THE INVENTION

The present invention is directed to a dispersion spun fluoropolymer fiber including a blend of non-melt processible, high molecular weight, polytetrafluoroethylene particles and perfluoroalkoxy (PFA) particles. The fluoropolymer fiber is prepared by forming an aqueous dispersion of perfluoroalkoxy particles and non-melt-processible polytetrafluoroethylene particles, the polytetrafluoroethylene particles having a standard specific gravity (SSG) of less than about 2.4. The ratio of polytetrafluoroethylene particles to perfluoroalkoxy particles in the aqueous dispersion may range from between about 6:1 to about 3:1, with specific ratios of 6:1, 5:1, 4:1, and 3:1. The aqueous dispersion of polytetrafluoroethylene particles and perfluoroalkoxy particles is mixed with an aqueous matrix polymer solution containing a matrix polymer selected from the group consisting of methylcellulose, hydroxyethylcellulose, methylhydroxypropylcellulose, hydroxypropylmethylcellulose, hydroxypropylcellulose, ethylcellulose and carboxymethylcellulose. The mixture is then extruded into a coagulation bath containing a concentration of ions which coagulate the matrix polymer to form an intermediate fiber structure which carries ionic species. Thereafter, the intermediate fiber structure is sintered to decompose the matrix polymer and coalesce the polytetrafluoroethylene particles and the perfluoroalkoxy particles into a blended fiber. The resulting, blended fluoropolymer fiber, which exhibits improved properties relative to 100% dispersion spun polytetrafluoroethylene fibers, is suitable for use in bearings, bushings, fabrics, belts, diaphragms, coatings, filters and seals.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a differential scanning calorimeter (DSC) scan showing the melting behavior of a dispersion spun, fluoropolymer fiber prepared in accordance with the method described herein.

FIG. 2 is a DSC scan showing the melting behavior of a 100% PTFE dispersion spun fiber.

FIG. 3 shows a heat flow cool down DSC scan of the fluoropolymer of FIG. 1

FIG. 4 shows a heat flow cool down DSC scan of the 100% PTFE dispersion spun fiber of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a dispersion spun fluoropolymer fiber including a blend of polytetrafluoroethylene particles and perfluoroalkoxy particles. The fluoropolymer fiber is prepared by forming an aqueous dispersion of perfluoroalkoxy particles and polytetrafluoroethylene particles, mixing the dispersion with an aqueous matrix polymer solution containing a matrix polymer, extruding the mixture into a coagulation bath and forming an intermediate fiber structure. The intermediate fiber structure is then sintered to decompose the matrix polymer and coalesce the polytetrafluoroethylene particles and the perfluoroalkoxy particles into a blended fiber.

The matrix spinning process of PTFE allows for the inclusion of an appreciable concentration of PFA into a fiber structure that has sufficient tensile properties for normal textile processing such as knitting and weaving. The inclusion of PFA into a matrix spun PTFE fiber, results in a true bicomponent fluoropolymer fiber with typical thermal capabilities (maximum continuous use temperature) of PTFE. Further, the inclusion of PFA into the fiber matrix provides a lower melt component to the fiber. This lower melt component when in a fabric structure provides a system to which a PFA film can be laminated at lower temperatures and pressures compared to 100% PTFE. An exemplary PFA laminated film product is described in U.S. Provisional Patent Application No. 61/635,721, filed on Apr. 19, 2012 and titled “Flexible Laminate Structure,” the entire contents of which are incorporated herein by reference.

Polytetrafluoroethylene

The polytetrafluoroethylene particles used in the dispersion employed in this invention are non-melt-processible particles of polytetrafluoroethylene (PTFE) including modified PTFE which is not melt-processible. Polytetrafluoroethylene (PTFE) refers to the polymerized tetrafluoroethylene by itself without any significant comonomer present. Modified PTFE refers to copolymers of TFE with such small concentrations of comonomer that the melting point of the resultant polymer is not substantially reduced below that of PTFE. The concentration of such comonomer is preferably less than 1 wt %, more preferably less than 0.5 wt %. The modified PTFE contains a small amount of comonomer modifier which improves film forming capability during baking (fusing), such as perfluoroolefin, notably hexafluoropropylene (HFP) or perfluoro(alkyl vinyl)ether (PAVE), where the alkyl group contains 1 to 5 carbon atoms, with perfluoro(ethyl vinyl)ether (PEVE) and perfluoro(propyl vinyl)ether (PPVE) being preferred. Chlorotrifluoroethylene (CTFE), perfluorobutyl ethylene (PFBE), or other monomer that introduces bulky side groups into the molecule are also included. The PTFE typically has a melt creep viscosity of at least 1×10⁹ Pa·s. The resins in the dispersion used in this invention when isolated and dried are non-melt-processible. Such high melt viscosity indicates that the PTFE does not flow in the molten state and therefore is non-melt-processible

By non-melt-processible, it is meant that no melt flow is detected when tested by the standard melt viscosity determining procedure for melt-processible polymers. This test is according to ASTM D-1238-00 modified as follows: The cylinder, orifice and piston tip are made of corrosion resistant alloy, Haynes Stellite 19, made by Haynes Stellite Co. The 5.0 g sample is charged to the 9.53 mm (0.375 inch) inside diameter cylinder which is maintained at 372° C. Five minutes after the sample is charged to the cylinder, it is extruded through a 2.10 mm (0.0825 inch diameter), 8.00 mm (0.315 inch) long square-edge orifice under a load (piston plus weight) of 5000 grams. This corresponds to a shear stress of 44.8 KPa (6.5 pounds per square inch). No melt extrudate is observed.

The polytetrafluoroethylene particles have a standard specific gravity (SSG) of less than 2.40, typically from about 2.14 to about 2.40, preferably less than about 2.30, and more preferably less than about 2.25. The SSG is generally inversely proportional to the molecular weight of PTFE or modified PTFE.

The fluoropolymer particles in the dispersion used in this invention preferably have a number average particle size of about 100 nm to about 400 nm, most preferably, about 120 nm to about 220 nm.

Perfluoroalkoxy

The perfluoroalkoxy particles used in the dispersion employed in this invention are melt-processible particles of perfluoroalkoxy (PFA). As used herein, the term “perfluoroalkoxy” refers to a copolymer of tetrafluoroethylene and perfluoroalkylvinylether. A suitable aqueous PFA dispersion is available from E. I. duPont de Nemours & Co., Wilmington, Del. under the name Teflon® PFA TE-7224. Teflon® PFA TE-7224 is a negatively charged, hydrophobic colloid, containing approximately 60% (by total weight) of 0.05 to 0.5 μm perfluoroalkoxy (PFA) resin particles suspended in water. A milky white liquid, Teflon® PFA TE-7224 also contains approximately 5% by weight of a nonionic wetting agent and stabilizer (based on the weight of the PFA solids). Viscosity at room temperature is approximately 20 cP. Nominal pH is 10. The resin in Teflon® PFA TE-7224 is a true thermoplastic and will exhibit some flow above its melting point. When properly processed, the PFA resin in Teflon® PFA TE-7224 exhibits retention of properties after service at 260° C. (500° F.), useful properties at −240° C. (−400° F.), chemical inertness to nearly all industrial chemicals and solvents, and low friction and antistick surfaces.

Spinning Composition and Matrix Polymers

The present invention provides a spinning composition useful for the dispersion spinning of non-melt-processible fluoropolymer fiber comprising a mixture of an aqueous solution of a matrix polymer and an aqueous dispersion of perfluoroalkoxy particles and non-melt-processible polytetrafluoroethylene particles having an SSG of less than about 2.40, typically from about 2.14 to about 2.40. In preferred embodiments the non-melt-processible polytetrafluoroethylene particles have an SSG of less than 2.30, and more preferably less than about 2.25.

The aqueous dispersion of perfluoroalkoxy particles and polytetrafluoroethylene particles is prepared by pouring the PTFE dispersion into a tote and adding the PFA dispersion to the PTFE dispersion. The dispersion is mixed mechanically in the tote for about an hour with slow agitation to avoid shear. The mixed dispersion is then loaded into a supply tank and put under vacuum. The aqueous dispersion includes, by weight, 75% PTFE and 25% PFA.

Matrix polymers used in the practice of the present invention may be polymers containing only hydrogen, carbon, oxygen and nitrogen that are soluble in aqueous solutions that may be coagulated or precipitated by a salt or a shift of pH. As taught in U.S. Pat. Nos. 3,655,853; 3,114,672; and 2,772,444 cellulose xanthate may be the soluble form of the matrix. However, the use of viscose in fiber forming suffers from serious disadvantages related to cost of manufacture, production time and environmental hazards. Alternatives to viscose forming have been developed and most recently a process using cellulosic ethers with a uniform degree of substitution of the matrix has been fully described in U.S. Pat. Nos. 5,762,846 and 5,820,984.

Cellulosic ether polymers are preferred since these polymers do not melt or soften below the temperature range in which most fluorinated olefinic polymers melt and the polymer decomposes into carbonaceous material on sintering. For example, such cellulosic polymers are methylcellulose, hydroxyethylcellulose, methylhydroxypropylcellulose, hydroxypropylmethylcellulose, hydroxypropylcellulose, ethylcellulose and carboxymethylcellulose. The cellulosic ethers preferred for use in this invention as a matrix polymer have a uniform degree of substitution, and are soluble in strong aqueous alkali hydroxide, but insoluble in near neutral pH water. By the term near neutral pH water is meant water having a pH from about 6 to 8. In addition, the matrix polymers used in the practice of the present invention have no softening or melting point. These polymers decompose at temperatures near the sintering temperature of the fiber providing requisite tensile strength until the fluoropolymer particles have coalesced such that the resultant fluoropolymer structure provides the necessary tensile strength.

In order to achieve useful coalesced fluoropolymer fibers, it is desirable to wash the intermediate fiber structure free of ions absorbed from the coagulation bath as well as to remove other impurities such as additives and/or dispersants that are present in the initial fluoropolymer dispersion and to remove materials that are detrimental to fiber sintering and/or the properties of the final, coalesced fluorinated polymer fiber.

As used herein, intermediate fiber structure means the extruded and coagulated mixture of the matrix polymer solution and the polymer particle dispersion. The intermediate fiber structure produced in the practice of the present invention, after washing in near neutral pH water to substantially remove ions and impurities, shows no substantial loss of strength or integrity, and may be worked, for example drawn at a modest draw ratio, and sintered to form a final, coalesced fluorinated polymer fiber or shaped article. The intermediate fiber structure produced by the present invention may be isolated, worked in subsequent processing or used for producing fabrics or batts as is known in this art.

In the practice of the present invention, the composition of the intermediate fiber structure has a cellulosic ether present as a minor constituent of the fiber solids, while the major constituents are perfluoroalkoxy and non-melt processible fluoropolymer particles having a weight in the intermediate fiber structure that may be from 3 to 20 times that of the matrix polymer.

The spinning compositions used in the process of the present invention are made by mixing an aqueous dispersion of fluorinated polymer particles with a solution of the matrix polymer. Aqueous dispersions of perfluoroalkoxy particles and non-melt processible polytetrafluoroethylene particles, such as those described above are used in the present process. The solutions of matrix polymer should be clear and of a viscosity that assures good mixing with the dispersion. Preferably the concentration of matrix polymer in the solution is from 3 to 10% by weight. These components are then mixed such that the ratio of the weight of the polymer particles to that of the matrix polymer in the intermediate fiber structure is from about 3 to 1 to about 20 to 1, and preferably about 9 to 1.

Example

Utilizing the method described herein, low shrink fluoropolymer fibers including, by weight, 75% PTFE and 25% PFA, were prepared as further described in Table 1. The spin mix was prepared from an aqueous dispersion of fluoropolymer particles containing PTFE dispersion obtained from by E. I. duPont de Nemours & Co. and the Dupont Company's PFA TE7233 dispersion. The matrix polymer utilized in the spin mix was CS Polymer (Hydroxypropyl Cellulose) obtained from Shin Etsu Chemical Industry Co. Tokyo, Japan.

TABLE 1 Tensile Elongation L- Strength at Break Shrinkage Draw Bobbin Denier Color (gf/denier) (%) (%) Ratio 3 420 19.06 1.16 64.15 9.037 420 1.30 74.49 5 360 19.58 1.20 61.49 9.650 360 1.15 47.66 6 374 17.41 1.15 59.15 7.256 374 1.09 50.99 7 370 18.57 1.14 52.93 8.134 370 1.19 49.48 Avg 381 18.66 1.17 57.54 8.519 7.0

A portion of the fluoropolymer fibers were processed into floc, and the melting behavior of the fluoropolymer fiber floc at the second heat was observed using a differential scanning calorimeter (DSC). The DSC scan results for the fluoropolymer fiber floc were compared to the melting behavior of a 100% dispersion spun PTFE fiber floc. Referring to FIG. 1, the DSC curve shows that the fluoropolymer fiber exhibited a PFA endotherm at 301° C. and a PTFE endotherm at 325° C., which clearly indicates the bicomponent nature of the fiber. FIG. 2, which depicts the DSC scan showing the melting behavior of a 100% PTFE dispersion spun fiber, exhibits a single endoderm at 327° C. An endoderm at 301° C. is absent. A cool down DSC scan of the fluoropolymer fiber is depicted at FIG. 3. The cool down scan shows that the fluoropolymer fiber exhibits a noticeable PFA exotherm at 284.7° C. and a PTFE exotherm at 311.08° C. The PFA exotherm is absent in FIG. 4, which depicts a cool down DSC scan of the 100% PTFE fiber.

The bonding strength of the fluoropolymer fibers was tested and compared to the bonding strength of dispersion spun 100% PTFE fibers. Each fluoropolymer fiber sample was twisted and then cut longer than the platen press. The fibers samples were twisted to avoid having the samples being encapsulated by the film laminate and providing erroneous results. Weight was added to each end of the samples to maintain tension. A PFA film was laid beneath the samples and another film was laid on top. The platen press was closed and heated until the two films laminate. At this point the press was opened, and the sample were cooled. Once cooled, one end of the samples was trimmed to remove the exposed fibers. The remaining sample was put into an instron, and the force to remove the fibers from the film laminate was measured. The force required to remove the fibers from between the PFA laminates was higher for the PTFE/PFA blended fiber than the 100% PTFE fiber.

As will be apparent to one skilled in the art, various modifications can be made within the scope of the aforesaid description. Such modifications being within the ability of one skilled in the art form a part of the present invention and are embraced by the claims below. 

It is claimed:
 1. A method of making a fluoropolymer fiber exhibiting two melting points comprising, forming a mixture of an aqueous dispersion of non-melt-processible polytetrafluoroethylene particles and perfluoroalkoxy particles with an aqueous solution of a matrix polymer, extruding the mixture into a coagulation bath and forming an intermediate fiber structure by coagulating the matrix polymer in the coagulation bath, and sintering the intermediate fiber structure to decompose the matrix polymer and coalesce the polytetrafluoroethylene particles and the perfluoroalkoxy particles into a blended fiber.
 2. The method according to claim 1 wherein the ratio of polytetrafluoroethylene particles to perfluoroalkoxy particles in the mixture is about 3 to
 1. 3. The method according to claim 1 wherein the two melting points include a first melting point attributable to the polytetrafluoroethylene particles and a second melting point attributable to the perfluoroalkoxy particles.
 4. A fluoropolymer fiber comprising a blend of non-melt-processible polytetrafluoroethylene particles and perfluoroalkoxy particles.
 5. The fluoropolymer fiber according to claim 4 including up to about 25% perfluoroalkoxy particles and up to about 75% polytetrafluoroethylene particles.
 6. The fluoropolymer fiber according to claim 4 having a maximum continuous use temperature of about 260° C.
 7. The fluoropolymer fiber according to claim 4 having a tenacity of at least about 1.1 gpd.
 8. The fluoropolymer fiber according to claim 4 having an elongation to break of about 60%.
 9. The fluoropolymer fiber according to claim 4 having two different melting points.
 10. The fluoropolymer fiber according to claim 9 wherein the two different melting points include a first melting point of about 301° C. and a second melting point of about 325° C.
 11. The fluoropolymer fiber according to claim 4 having two different crystallization points.
 12. The fluoropolymer fiber according to claim 11 wherein the two different crystallization points include a first crystallization point of about 285° C. and a second crystallization point of about 312° C.
 13. A fluoropolymer fiber comprising a continuous multifilament yarn including polytetrafluoroethylene particles and perfluoroalkoxy particles.
 14. The fluoropolymer fiber according to claim 13 wherein the polytetrafluoroethylene particles and perfluoroalkoxy particles are coalesced and entangled.
 15. The fluoropolymer fiber according to claim 13 further comprising a matrix polymer.
 16. The fluoropolymer fiber according to claim 13 wherein the fluoropolymer fiber exhibits two different endotherms when observed using a differential scanning calorimeter.
 17. The fluoropolymer according to claim 16 wherein the two different endotherms include a perfluoroalkoxy endotherm at about 301° C.
 18. The fluoropolymer according to claim 16 wherein the two different endotherms include a polytetrafluoroethylene endotherm at about 325° C.
 19. The fluoropolymer fiber according to claim 13 wherein the fluoropolymer fiber exhibits two different exotherms when observed using a differential scanning calorimeter.
 20. The fluoropolymer according to claim 19 wherein the two different exotherms include a perfluoroalkoxy exotherm at about 287.7° C. and a polytetrafluoroethylene exotherm at about 311.08° C. 